The present invention relates to monitoring aspects of a plasma enhanced chemical vapor deposition (PECVD) chamber, including the alignment and spacing of a susceptor relative to a gas discharge head.
In a PECVD chamber, a semiconductor wafer is placed on a susceptor underneath a gas discharge head. Typically, an RF potential is established between the susceptor and the gas discharge head to enhance the deposition of chemicals which are introduced into the chamber in vapor form through the gas discharge head. In order to insure even deposition or reactions across the wafer surface, it is essential to have the susceptor parallel to and properly aligned with the gas discharge head. Even when properly aligned, the proper distance is also important, with different processes requiring different distances as a method of controlling the amount or power of the plasma generated.
Typically, a susceptor is manually adjusted before its first use, using a pair of control knobs which connect to perpendicular rods to adjust the level of the susceptor. The knobs, which extend through the chamber walls, are adjusted and the level of the susceptor can be measured with any of a number of different mechanical calibration tools. In one simple version, a flat bar is placed across ledges in a chamber and the spacing between the bar and the top of the susceptor is visually observed and adjusted to be even. Another necessary adjustment is the calibration height of the susceptor. Typically, the susceptor is mounted on fingers or a pedestal which are controlled by a motor to raise and lower the susceptor. This is done to both control the distance to be different for different processes, and to lower the susceptor for loading and unloading of wafers. Typically, wafers are loaded through a slot in the side of the chamber when the susceptor is lowered, and then the susceptor is raised up to the desired level under the control of electronic circuitry controlling the motor, which is itself guided by a software routine governing the process being run. Again, typically, the height of the susceptor is manually measured relative to the gas discharge head for a zero position of the electronic circuitry, with the calibration value being entered into the program for adjustment.
A final determination of whether the adjustment has properly aligned the susceptor is done by running test wafers through a deposition process, and then measuring the uniformity of the film deposited on the wafer. In one method, this is done by measuring 49 different points in a pattern across the wafer, and then dividing the standard deviation by the average thickness. A currently acceptable industry standard is that this quotient be no greater than 1.5%.
Typically, the entire alignment process, including running the test wafers, may take around 4-5 hours. This alignment not only needs to be done initially, but also periodically to insure that the susceptor stays aligned. Due to the periodic raising and lowering of the susceptor and the chemical reactions to which the chamber is subjected at varying pressures and temperatures, the susceptor typically becomes unaligned over a period of time, thus requiring periodic realignment. Such realignment requires stopping the process, cooling the chamber and breaking the vacuum, and opening the lid of the chamber in order to mechanically measure the alignment.
Accordingly, it would be desirable to have an alignment system which could be performed in a shorter time period and which would not require opening the chamber.